Solar Particle Events and Evaluation of Their Effects During Spacecraft Design Seps in Environment Specifications

SOLAR PARTICLE EVENTS AND EVALUATION OF THEIR EFFECTS DURING SPACECRAFT DESIGN Piers Jiggens ([email protected]), Eamonn Daly, Hugh Evans, Alain Hilgers Spacecraft Environment and Effects Section, ESA/ESTEC, Noordwijk, The Netherlands (http://space-env.esa.int) THE SEP ENVIRONMENT MODELLING SEP FLUXES Solar Energetic Particles (SEPs) arrive in highly sporadic concentrations known as Solar Particle In order to model the SEP environment for mission specifications a Events (SPEs). SPEs are characterised by enhancements of several orders of magnitude above statistical or data-driven approach is applied rather than physics- background levels for protons of energies from less than 1 MeV to over 1 GeV in extreme cases. These based models of particle acceleration. This is because the short extreme particle storms are known as Ground Level Enhancements (GLEs) as they can be detected by term variability (space weather) is not important for spacecraft ground-based neutron monitors with secondary neutrons emissions resulting from high energy design wherein the goal is to design a spacecraft whose components particles (with sufficient magnetic rigidity to penetrate through the geomagnetic field) interacting do not fail as a result of the effects of particle radiation without over with the Earth’s upper atmosphere. SPEs are not restricted only to protons but also electrons and -engineering. Steps to derive and apply an SEP environment model significant level of alpha particles and heavier ions. for mission specification is shown (top right figure). Since the beginning of the space age over 40 years of space particle radiation have been recorded at Well known models of the environment include the JPL model [1] altitudes sufficient for fluxes to be un-attenuated by the Earth’s magnetic field down to energies of 5 and the ESP/PSYCHIC models [2,3]. For cumulative dose effects a MeV/nucleon. As well resolved data with good coverage is only available for protons and helium probabilistic model is defined based on statistical distributions fit heavy ions fluxes must be extrapolated from helium fluxes based on a subset of SPE observations. to SEP flux. The JPL approach performs a monte-carlo analysis SPEs can result from both solar flares and coronal based on the distribution of SPE fluences (applying a normal mass ejections (CMEs) but the latter is a distribution to the logarithms) whilst the ESP/PSYCHIC method requirement to produce the largest flux performs an enhancements seen at the Earth. The largest SPEs analytical derive mission specifications as a handful of events extrapolation can contribute over 90% of the total fluence based on the SEP yearly fluence distribution (applying a observed. This highlights the difficulty in modelling lognormal distribution). For worst-case SPE peak fluxes and this highly variable environment. In spacecraft fluences the standard method from CREME96 [4] is to take an design the customer specifies a confidence level for observed worst case (for the October 1989 SPE) but statistical the certainty that the flux will not be exceeded approaches are also available. The new SAPPHIRE (Solar commonly in the range of 90% to 95%. The result is Accumulated and Peak Proton and Heavy Ion Radiation that the SEP flux observed by missions is often well Environment) model being developed as part of ESA R&D below the levels given in the specification. developments aims to derive all SEP environment outputs Specifications include geomagnetic attenuation of applying a self-consistent statistical approach (with an particle fluences given by model outputs so the exponential cut-off power law applied to the SPE peak fluxes or higher the geomagnetic latitude the lower the fluences) [5]. magnetic shielding and higher the calculated flux. Comparisons of the different approaches for mission cumulative SEPs are important contributors to ionising dose fluences for an example 5-year 95% confidence level (bottom effects resulting in degradation of electronics left figure) identify large differences between model outputs performance due to energy deposited in semi- especially pronounced at higher energies. These differences conductors and non-ionising dose effects resulting result from different data, processing, modelling approaches in degradation in opto-electronic components due to and distributions. Two versions of the ESP/PSYCHIC model atomic displacements. Ionising radiation can also show differences resulting from the application of spectral interfere with electronics performance resulting in fittings. Outputs of different approaches for the worst case SPE single event effects (SEEs). fluence (worst week see bottom left figure). [12] SEPS IN ENVIRONMENT SPECIFICATIONS SUPPORTING MISSION OPERATIONS For the derivation of effects quantities in an environment specification the differential flux is calculated and then passed to the An environment specification is intended for general mission design which is tailored for appropriate tool (examples given below): the system-level requirements of the specific mission. However, some instruments or operations require additional support for effects which can impact their contribution to Total Ionising Dose: SHIELDOSE-2(Q) [6] or MULASSIS (Monte-Carlo) [7] mission success. Non-ionising Dose: Spenvis [8] NIEL tool or MC-SCREAM (Monte-Carlo) One well documented end user is the human spaceflight community. Due to the Solar cell degradation: EQFLUX enhanced radiation dose exposure during EVAs (Extra-Vehicular Activities) it can be SEE rate: LET with interaction cross-sections or GEMAT (Geant4 [10]) important to know the present and forecasted SEP Cumulative dose effect (e.g. TID, NID and solar cell degradation) calculation environment. Although this is less important at ISS tools are applied to mission cumulative fluxes. For TID in a GEO orbit the (International Space Station) altitude/geomagnetic dominant contribution to the total for nominal shielding thicknesses (~4 mm latitude it will be an important consideration for Al) are the trapped electrons which are a factor 3 higher than solar protons even mission planning in the future if agencies are sending when taking a 90% confidence level (see top right figure). However, solar humans into deep space [11]. Even inside a spacecraft protons are increasingly important as shielding thickness is increased although astronauts may move into more highly protected areas in the case of LEO trapped protons are likely to dominate in this region. during especially harsh SPE events when they occur. For NID and solar cell degradation (and TID for high shielding levels) in LEO Another area where SPE observation and forecast can (sun-synchronous at 725 km) solar protons compete with trapped protons for be of critical importance is for optical instruments. In terms of operations sensor dominant contribution. The hard spectra of the trapped protons when passing interference can disrupt the operation of star trackers. This was seen as a critical for the through the South Atlantic Anomaly (SAA) results in much higher doses for orbit insertion manoeuvres on the Gaia mission and the Launch and Early Orbit Phase high shielding but solar protons give a significant contribution for solar cell (LEOP) and transfer to the L2 Lagrange point were supported by the SSA programme degradation at nominal cover glass thicknesses despite attenuation of SEPs due with information on the environment. Sharp increases in the SEP environment can also to geomagnetic shielding (see middle right figure). be important for launch operations due to increased SEE rates if the launch vehicle SEE rate calculations are component-specific and a specification provides flux reaches sufficiently high geomagnetic latitudes. In addition to a solar activity go/no-go as a function of Linear Energy Transfer (LET) for characterisation of the criteria for launch analyses are performed by environment experts to determine the tolerance and on-board correction required applicable for heavier particles critical energy range and flux threshold for SPEs to concern mission operations. It is capable of causing upsets through direct ionisation. Lighter particles such as important to note that the aim is to provide realistic values and reliable forecasts to avoid protons require nuclear interactions and are characterised by an SEE cross- unnecessary additional section derived from testing. mission costs in addition to During large SPEs, SEPs are ensuring mission success is dominant in terms of SEE rate not imperilled by particle compared to galactic cosmic radiation impacts. rays (GCRs) as long as the In a future space weather spacecraft is not highly system many space missions shielded by the Earth’s designed to characterise the magnetosphere (bottom right space weather environment figure). will rely on optical For missions travelling closer instrumentation. Clearly to the Sun (such as ESA’s such measurements are BepiColombo mission most affected by SEPs travelling to Mercury) during times of high activity probabilistic solar protons which are of greatest fluxes are scaled based on the heliocentric distance (see bottom left figure). As a interest to the space result SEP specified doses and degradations are far in excess of those included in weather forecaster. specifications for LEO and GEO. Robustness in design of such instrumentation such Note that observed SEP effects are likely to be far lower than those included in as a reduction in exposure time can limit the affect of sensor interference such as seen

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